This invention relates to a radiation-sensitive record disc; e.g., an optical disc or video disc. Recently, radiation-sensitive record discs have been widely used for recording and/or reproducing information due to their desirable operational characteristics. For example, radiation-sensitive record discs have the following advantages:
1. High-density recording is provided.
2. Information can be recorded and reproduced without contact between the recording/reading head and the discs. Thus, the surface of the disc will not be scratched and its recording integrity adversely effected.
3. Random access is provided at high speeds.
4. The recording can be preserved for a long period of time without information loss.
5. The recording system is simple in construction and operation.
Because of these advantages, radiation-sensitive record discs are widely used for information recording-reproducing purposes or solely for reproducing purposes. Radiation-sensitive record discs used solely for reproducing purposes may be used, for example, in home video disc systems, business video disc systems, and auto pulse code modification (PCM) disc systems. On the other hand, radiation-sensitive record discs used for recording-reproducing purposes may be used, for example, in document retrieval files, TV video files (i.e., broadcast systems) and as a memory for general purpose computer systems.
The conventional radiation-sensitive record disc as shown in U.S. Pat. No. 4,074,282, comprises two substrates (FIG. 2) each formed, for example, of a transparent plastic plate. Formed directly on one surface of each substrate is an optical information recording layer consisting of an energy-absorbing layer or a light reflective layer. The two substrates of this construction are disposed to face each other such that each recording layer is positioned therebetween. Each recording layer is bonded to a respective substrate with spacers disposed therebetween so as to provide a space between each recording layer.
The substrates of this conventional system must be formed with a relatively small thickness since the writing/reading beam must pass through the substrate without distortion. That is, the use of a substrate having small thickness prevents beam energfy loss and refraction of light as the beam passes through the substrate. Due to the substrate's thickness and transparency requirements, the material used will lack stiffness. However, the air or gas within the space between the substrates will expand or contract as the surrounding heat or atmospheric pressure changes. As a result, the radiation-sensitive record disc will bend or undulate in accordance with this expansion or contraction. Any bending or undulation will cause defocusing of the reading/writing beam resulting in inaccuracies in processing the information. In extreme cases, the record disc and the optical head will eventually contact each other, thereby preventing any processing of the information.